It has been known for a long time that the hormonal environment of a person influences bone marrow function. However, the precise molecular mechanism by which this occurs is not known. Epigenetic changes, such as the attachment of a methyl group to DNA and the alteration of histone proteins can regulate the expression of genes that are necessary for stem cell differentiation. This proposal utilizes an animal model that expresses DNMT3B7, a truncated DNA methyltransferase 3B isoform that is commonly expressed in human cancers, to interrogate whether DNA methylation changes can influence the development of blood cells. To test the influence of DNMT3B7 expression on blood cell development, DNMT3B7-expressing fetal liver cells or wild-type E14.5 fetal liver cells were transplanted into recipient animals. Interestingly, female recipients failed to achieve normal peripheral blood counts in the first two months after undergoing transplantation with DNMT3B7-expressing cells, but male recipients had normal blood counts. Transplantation into females that lacked female hormones demonstrated loss of the observed effect, suggesting that the female hormonal milieu is suppressive to DNMT3B7-expressing stem cell function. Thus, we have found that DNMT3B7 expression uncovers a striking influence of gender i.e., hormonal milieu, on stem cell function. I hypothesize that the expression of DNMT3B7 renders HSCs more sensitive to the hormonal milieu in female mice by altering DNA methylation patterns, and thus the expression of genes associated with HSC self- renewal and differentiation. I will use three specific aims to test my hypothesis: (1) Test the impact of DNMT3B7 expression on hematopoietic stem and progenitor cells regarding (a) induced epigenetic changes and (b) their in vitro colony forming potential in the presence and absence of sex-specific hormones; (2) Measure the influence of sex hormones on the homing of HSCs to the bone marrow; (3) Examine the mechanism by which DNMT3B7 expression renders HSCs responsive to female hormones. The proposed work will give us more insight into the mechanisms that promote HSC engraftment and reconstitution in the presence of sex-specific hormones and therefore could have a significant impact on clinical stem cell transplantation.